This invention relates to an improved composition of a phenol-aldehyde resin containing a particular type of salt and to the cellulosic fibrous battery separator impregnated with the composition.
Permeable cellulosic fibrous battery separators are used extensively in batteries of either the conventional wet lead-acid type or the so-called "dry charge" lead-acid type. It is necessary that the fibrous structures thereof be protected from attack by the acid employed in the batteries. Therefore, when cellulosic fibrous battery separators are employed, they consist of cellulosic sheets impregnated with a phenol-aldehyde type of resin, which, when advanced to the infusible state, protects the fibers of the cellulosic sheet from attack by the acid.
The phenolic resin employed in battery separators are usually water base resole liquid resins or solvent-containing "varnishes". In order to produce satisfactory battery separator properties the resins are generally supplied in a pH range of about 7.0-8.0, which is accomplished by neutralizing most of the basic catalyst employed in forming the resin. According to the acid employed for neutralization, either an insoluble salt or a soluble salt may be present.
The insoluble salt is removed by filtration, while the soluble salt is left in the resin. In the latter case, the choice of relatively inexpensive acids is quite limited. Thus, hydrochloric acid cannot be used because chloride has a deleterious effect on batteries. Acetic acid is volatile and may come off during cure of the separator, causing the pH to rise and thus producing a dark, brittle separator.
Sulfuric acid is used and the commonly formed salt is sodium sulfate, since the resole is often catalyzed with sodium hydroxide. While this salt is generally water soluble in the resole, it will precipitate out as the decahydrate if the resin solids are too high, or if the temperature of the resin becomes too low. Resin solids level can be adjusted to prevent precipitation due to that factor, but precipitation due to low temperature is a cold weather problem and generally happens in storage tanks of the battery separator producer. Since there are usually no facilities for heating such tank, the resin may become unsatisfactory for use due to the precipitate.
Thus, it would be beneficial to neutralize with an acid having none of the above problems. A class of hydroxy organic acids were found to be beneficial. The hydroxy acids are selected from the group consisting of citric, tartaric and lactic or mixtures thereof. Not only does it eliminate the salting out problem, but, surprisingly the separators made from the resin containing polyacid salts had improved oxidation resistance, an important property, over a similar base resin containing sodium sulfate salt.